Results: the asus router was 40% idle, with the limit of packets reached ( ~ 15 Mbit/s );
the rb750Gl was completely used, idle less than 1% with a bandwidth of ~175 Mbit/sec and ~ 7.5K packets sent and ~ 17K packets received ;
the tplink was 20% idle, with a total of ~ 190 Mbit/s , ~18K packets received and ~8K packets sent.

That is impressive, considering that the mikrotik solution is commercial and was expected to provide way more bandwidth (with way less firewall rules!),
instead the openwrt solution is able to overload it even without the so 'blamed' hw accel https://dev.openwrt.org/ticket/11779 . Great job openwrt!

For whatever SOHO solution that has no so big traffic between two logical networks (for example, less than 50 Mbit on average),
the wdr3600 plus openwrt and thirdy party software (mwan3) is a great solution even for business.

Restoration of original firmware (without modifying U-Boot).
The firmware must be 8,126,464 octets which correspond to a firmware without U-Boot.
Today, firmwares TP-Link : wdr3600v1_en_3_13_23_up(120820), wdr3600v1_en_3_13_26_up(130129) and
wdr3600v1_en_3_13_31_up(130320) come without U-Boot.
The version used in the example is wdr3600v1_en_3_13_31_up(130320) on the WDR3600 v1.3

Bootloader Mods

U-Boot 1.1.4 modification for routers

Forum member pepe2k made a modification of U-Boot 1.1.4 for Qualcomm Atheros SoCs based devices (the project is still being developed, so new devices and SoCs will be supported in the future). Up to date information, binary images and sources can be found on official GitHub repository.

This modification started from wr703n-uboot-with-web-failsafe project, but supports more devices, all modern web browsers, has a lot of improvements and other modifications (like U-Boot NetConsole, custom commands, overclocking possibilities etc.).

Hardware Modifications

USB Modification

It turns out that the GL850G chipset used by the TP-Link in WDR3600/4300/4900 models can handle up to four ports.

Analysing the router's PCB it appears that pins 8(D-), 9(D+), 11(D-) and 12(D+) are unused.
Aditionaly each factory USB port has separate power section.

GND is at the TP7 pin point. +5 V was taken directly from the MOSFET. +5 V can be also taken from TP5 near the power switch. The Maplin part N75HL is perfect for adding these externally and can be mounted under one side once a small hole has been drilled in the case.